sedimentary

Chert (River Pebble)

Silicon Dioxide (SiO2)

Hardness: 7 (Mohs scale); Color: Gray, tan, orange, and black mottling; Luster: Waxy to dull; Breakage: Conchoidal fracture; Structure: Microcrystalline.

Hardness: 7 (Mohs scale)
Color: Gray, tan, orange, and black mottling
Luster: Waxy to dull

Identified July 14, 2026More sedimentary →

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Physical properties

Hardness: 7 (Mohs scale); Color: Gray, tan, orange, and black mottling; Luster: Waxy to dull; Breakage: Conchoidal fracture; Structure: Microcrystalline.

Formation & geological history

Formed from the accumulation of silica-rich organic remains or chemical precipitation in marine or freshwater environments. This specimen has been smoothed and rounded through water erosion in a riverbed.

Uses & applications

Primarily used in construction (road gravel), as an abrasive, and historically for stone tools due to its sharp edges when broken. Water-worn pebbles are often collected as aquarium stone or garden accents.

Geological facts

The orange and red hues are often caused by the oxidation of iron impurities (hematite or goethite) within the silica matrix. Chert was one of the most important minerals for early humans to create arrowheads and scrapers.

Field identification & locations

Identify in the field by its hardness (it can scratch glass), its smooth, waxy feel, and its lack of visible grains. It is extremely common in riverbeds and gravel pits worldwide.